Notes from a French magazine called Ca m'interesse from January 2014. One article talked about choices, habits, and routines. Some findings are applicable to game design.
Richard Weiseman, a psychologist, followed 3000 people taking New Year resolutions. Half say they will hold their resolution on day 1. 12% manage it after one year. Weiseman gives tips on how to keep your resolutions, such as telling others about your resolution (to put peer pressure on yourself), avoiding previous (failed) resolutions (to prevent frustration), breaking it in smaller achievable steps (to prevent hopelessness), and giving yourself rewards for achieving these steps (positive reinforcement). It's been covered elsewhere too.
Based on previous research, Philippa Lally, another psychologist, suggests that it takes around two months working effortfully on new behavior to turn it into a habit.
Future hypothetical rewards (e.g. slim body in a month) are higher to accept than instant gratification (e.g. eat tasty food right now) because they require an effort of imagination. Solution: make future rewards more visible.
Baba Shiv, professor of marketing, asked 165 undergrads to pick either a chocolate cake or a fruit salad. The chocolate cake has a positive affect and negative cognition, ie it's emotionally appealing but you know you should not take it. The fruit salad is the opposite: negative affect and positive cognition, ie it's the rational healthy choice. In both groups, participants were asked to remember a number throughout the experiment. One group was given a 2-digit number (low cognitive load, ie high processing resources) and the other a 7-digit number (high cognitive load, ie low processing resources). 41% of the 2-digit group picked the tasty cake vs 63% in the 7-digit group. Under heavy cognitive load, people are more likely to choose options that are immediately pleasing. As a side note, when the presentation of the choice was through photos rather than the actual items, the difference between the two groups disappeared.
If I perceive a task as more difficult, I will expect a higher reward. It is not just about being strong-minded, but also about my perception of the task's difficulty.